randomize the serialization order of control frames (#4069)

* randomize the serialization order of control frames

* add comment for packetPacker.appendPacketPayload

packet_packer.go

@@ -1,9 +1,13 @@
 package quic
 
 import (
+	crand "crypto/rand"
+	"encoding/binary"
 	"errors"
 	"fmt"
 
+	"golang.org/x/exp/rand"
+
 	"github.com/quic-go/quic-go/internal/ackhandler"
 	"github.com/quic-go/quic-go/internal/handshake"
 	"github.com/quic-go/quic-go/internal/protocol"
@@ -122,6 +126,7 @@ type packetPacker struct {
 	acks                ackFrameSource
 	datagramQueue       *datagramQueue
 	retransmissionQueue *retransmissionQueue
+	rand                rand.Rand
 
 	numNonAckElicitingAcks int
 }
@@ -140,6 +145,9 @@ func newPacketPacker(
 	datagramQueue *datagramQueue,
 	perspective protocol.Perspective,
 ) *packetPacker {
+	var b [8]byte
+	_, _ = crand.Read(b[:])
+
 	return &packetPacker{
 		cryptoSetup:         cryptoSetup,
 		getDestConnID:       getDestConnID,
@@ -151,6 +159,7 @@ func newPacketPacker(
 		perspective:         perspective,
 		framer:              framer,
 		acks:                acks,
+		rand:                *rand.New(rand.NewSource(binary.BigEndian.Uint64(b[:]))),
 		pnManager:           packetNumberManager,
 	}
 }
@@ -832,6 +841,8 @@ func (p *packetPacker) appendShortHeaderPacket(
 	}, nil
 }
 
+// appendPacketPayload serializes the payload of a packet into the raw byte slice.
+// It modifies the order of payload.frames.
 func (p *packetPacker) appendPacketPayload(raw []byte, pl payload, paddingLen protocol.ByteCount, v protocol.VersionNumber) ([]byte, error) {
 	payloadOffset := len(raw)
 	if pl.ack != nil {
@@ -844,6 +855,11 @@ func (p *packetPacker) appendPacketPayload(raw []byte, pl payload, paddingLen pr
 	if paddingLen > 0 {
 		raw = append(raw, make([]byte, paddingLen)...)
 	}
+	// Randomize the order of the control frames.
+	// This makes sure that the receiver doesn't rely on the order in which frames are packed.
+	if len(pl.frames) > 1 {
+		p.rand.Shuffle(len(pl.frames), func(i, j int) { pl.frames[i], pl.frames[j] = pl.frames[j], pl.frames[i] })
+	}
 	for _, f := range pl.frames {
 		var err error
 		raw, err = f.Frame.Append(raw, v)